The present invention relates to flexographic printing. In particular, it relates to devices for forming flexographic printing plates.
"Flexography" is a term that broadly applies to a printing format which uses a flexible substrate bearing an elastomeric or rubbery relief printing surface.
A "composite" film is one with multiple layers, the adjacent layers being formed from different materials.
"Elastomer" and "elastomeric" refer to rubbers or polymers having resiliency properties similar to those of rubber. In particular, elastomers are capable of being substantially elongated and then returned to their original dimensions upon release of the stress elongating the elastomer. In all cases an elastomer must be able to undergo at least 10% elongation (at a thickness of 0.5 mm) and return to its original dimensions after being held at that elongation for 2 seconds and after being allowed 1 minute relaxation time.
More typically an elastomer can undergo 25% elongation without exceeding its elastic limit. In some cases an elastomer can undergo elongation to as much as 300% or more of its original dimensions without tearing or exceeding the elastic limit of the composition. Elastomers are typically defined to reflect this elasticity as in ASTM Designation D883-866 as a macro-molecular material that at room temperature returns rapidly to approximately its initial dimensions and shape after substantial deformation by a weak stress and release of the stress. ASTM Designation D012-87 can be an appropriate procedure for testing rubber properties in tension to evaluate elastomeric properties. Generally, such compositions include relatively high molecular weight compounds which, upon curing, form an integrated network or structure. The curing may be by a variety of means, including: the use of chemical curing agents, catalysts, and/or irradiation.
The first flexographic printing plates were produced from natural or synthetic rubber compositions which were chemically cured under heat and pressure in a mold using conventional rubber curatives such as mercapto compounds. Recently, photopolymer elastomeric compositions (elastomer containing compositions curable upon exposure to actinic radiation) useful to produce relief printing plates have been described. For example, U.S. Pat. Nos. 4,369,246 and 4,423,135 describe solvent-insoluble, elastomeric printing relief plates which are formed by applying to a flexible polymer film substrate layer a layer of a photosensitive composition. A cover sheet is laminated to a surface of the photo-sensitive layer to form a flexible film. A Flexographic plate is then formed by stripping off the cover sheet, imagewise exposing the composition layer to actinic radiation through the film, and removing the film and unexposed areas of the composition layer by solvent washing. Another known plate forming process is similar to the process described above, except that the relief areas of the plate are produced by washing out the areas not exposed to radiation with an aqueous developer.
With both the aqueous and solvent washing methods described above, multiple processing steps are required which are time consuming and are therefore expensive. In addition, these technologies produce potentially toxic by-product wastes when forming flexographic plates.
Due to the limitations of the above-mentioned prior art methods of forming flexographic printing plates, a new family of radiation curable polyurethane elastomers was developed which has the advantage of being useful to form flexographic printing plates without requiring the use of solvents or aqueous wash systems. An application having the title Flexographic Printing Plate Compositions, Ser. No. 558,301 filed on Jul. 26, 1990, has been made for the above-mentioned flexographic printing plate compositions by John Martens, one of the co-inventors of the present invention.
A flexographic printing plate 11 formed by a compound disclosed in the Martens composition application is shown in cross-section in FIG. 1. The printing plate 11 has a first base layer 15 which may be a polymer or a metal. Preferred polymers include polyester, polycarbonate, or polyimide, for example. The base material can also be made of steel or aluminum, for example. The base material is selected to be tear resistent and must have a fairly high melt point, for example, above the melt temperature of the next layer formed on the base layer. A second substantially uniform layer 17 is applied onto the base layer. The second layer is formed of a radiation curable polymer such as a polyurethane elastomer. The elastomer layer may be adhesively or chemically bonded to the base material (not shown). The base layer in combination with the radiation curable polymer together forms a "flexible film."
The flexible film is next exposed to actinic radiation from a surface 23 of the base layer 15 to chemically cross-link and cure a portion of the second layer 17 to form a floor layer 25 as described in an application having the title Flexographic Printing Plate Process, by John Martens, a co-inventor of the present invention, Ser. No. 558,301, filed Jul. 26, 1990, hereinafter referred to as the Martens "process" reference. A portion of the film is imagewise exposed to actinic radiation from the opposite surface 27 and a visible image appears in the exposed areas of the radiation curable layer.
The portions 29 of the flexible film exposed to radiation chemically cross-link and cure. The cured portions have an elevated melting temperature, and are insoluble in flexographic printing inks under normal conditions. "Normal" conditions include printing plate temperatures of between about 12 degrees Centigrade and 31 degrees Centigrade.
An absorbent material is selected having a melt temperature exceeding the melt temperature of the unexposed or "uncured" portions of the radiation curable composition and having good tear resistance at the same operating temperatures. Preferably, the selected material withstands temperatures required to process the film at an unsteady state, for example, where the plate does not reach a state of thermal equilibrium during heating. The absorbent sheet contacts a surface 27 of the radiation curable layer of the heated flexible film, o and absorbs the liquified portions 31 of the elastomer off of the upper surface, forming a flexographic printing plate 11. As described in the Martens "process" application, it is necessary to bring the most preferred printing plate and a sheet of the most preferred absorbent material into contact three times to remove the liquified, uncured elastomer.
The printing plates as described in the Martens et al. "process" application are heated to between 40 and 200 degrees Centigrade and while in contact with the absorbent layer, the uncured composition liquifies and flows into the absorbent layer. It is preferred that most of the composition in the uncured areas be removed from the flexible substrate and e absorbed into the absorbent material.
It is also desirable that the elevated temperatures used to cause the uncured composition to flow (reduce its viscosity, or exceed its softening temperature, e.g., see ASTM designation D 1238-86) into and be absorbed by the absorbent layer should not distort the flexible substrate or the hardened composition by more than 2% in any surface dimension. The actual temperatures will vary with the specific substrate and composition used.
Following curing of portions of the elastomeric layer, the uncured portions are removed to reveal a raised structure which becomes the printing surface of the plate. The "relief" portions are the raised portions of the flexographic printing plate which are coated with ink and which contact print media. The relief structure is bound to the support layer by physical and/or chemical action and remains in place during the printing process through repeat impressions. The exposed surface of the cured elastomeric layer therefore becomes the ink receiving surface which both receives ink from the inking row and transfers it to the substrate to printed during the printing process.
The hardening or curing step of the process (by irradiation) can also act to increase the adhesion of the composition to the substrate. This can be by direct adhesion of the curing composition to the substrate, for example, either chemical adhesion by reaction or physical adherence to surface structure on the flexible layer. The primer layer may be photosensitive or photoinsensitive to aid in this adherence effect. Flexographic plates having a layer for forming relief images are preferably formed from polymers which have a low melt temperature, a high melt index, are radiation curable, and are elastomeric polyurethane compositions. The composition should have sufficient resistance to swelling after curing. The cured elastomer compositions should be particularly resistant to increases in volume and swelling when immersed in flexographic inks, such as water based inks utilized in the flexographic printing process. This low swell feature is critical to the ultimate printing quality achieved with the printing plate because swelling causes the dimensions of the relief image to enlarge.
The press life or wear life of the plate on press (the number of impressions until failure occurs) may also be greatly reduced by swelling which may result in a loss of physical strength of the cured elastomer composition. Radiation curable polyurethane printing plates cure or cross-link when exposed to actinic radiation without the need of additional vinyl containing monomeric additives, such as monomers containing more than one vinyl group per molecule. Using a curing process which does not require the use of solvents avoids the harmful effects on the environment caused by pollution.
Radiation curable elastomer coated sheet material is resistant to cold flow during storage. Elastomers which undergo too much cold flow result in printing plates having unacceptable variation in plate thickness. The radiation curable polyurethane elastomers have good to superior resistance to cold flow prior to irradiation induced cross-linking or curing.
The use of flexographic printing plates is popular because the process is a "direct" printing process. The printing plate and its temporarily bound ink layer are in direct contact with the substrate, for example, the paper or film being printed. Before the Martens inventions, no devices existed to process flexographic printing plates by liquefying a portion of the flexible sheet, and removing the liquefied portions with an absorbent material.